In LTE (Long Term Evolution) Advanced of 3GPP (Third Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiplexing Access) using MU-MIMO (multi-user multiple-input multiple-output) has been proposed. In MU-MIMO downlink transmission, a base station is able to not only communicate with multiple mobile communication terminals, but also to transmit different data streams (layers) simultaneously to a mobile communication terminal.
In addition, in LTE Advanced, a reception technique for mobile communication terminals called as interference rejection combining has been discussed. Interference rejection combining (IRC) is a technique for downlink communication, in which a mobile communication terminal gives weights to signals obtained by reception antennas so as to suppress interference to the desired electric wave beam from the visited base station (desired base station) caused by interfering electric wave beams from interfering base stations at the mobile communication terminal. IRC improves the reception quality of desired signals on a desired electric wave beam especially in a case in which a mobile communication terminal 10 is located near the boundary of a visited cell 1a (cell of the desired base station 1) and receives strong interfering electric wave beams from another base station 2 (interfering base station) as shown in FIG. 1. In FIG. 1, reference symbol 2a denotes the cell of the interfering base station 2. In addition, in FIG. 1, a general shape of a beam 1b generated at the desired base station 1, and a general shape of the beam 2b generated at the interfering base station 2 are illustrated. A part of the beam 2b generated at the interfering base station 2, i.e., a part of a beam for downlink channels for other mobile communication terminals (for example, a mobile communication terminal 12) causes an interfering signal for the mobile communication terminal 10.
IRC is described in, for example, Patent Document 1, Non-patent Document 1, and Non-patent Document 2.
In an IRC reception technique, reception weights WMMSE, i for a mobile communication terminal that receives signals can be calculated with the use of Equation (1) below derived from an MMSE (minimum mean-square-error) algorithm.
                              W                      MMSE            ,            i                          =                                                            P                s                            ⁡                              (                                                      H                    i                                    ⁢                                      W                                          TX                      ,                      i                                                                      )                                      H                    ⁢                                    (                                                                    ∑                                          i                      =                      1                                                              N                      UE                                                        ⁢                                                                                    P                        s                                            ⁡                                              (                                                                              H                            i                                                    ⁢                                                      W                                                          TX                              ,                              i                                                                                                      )                                                              ⁢                                                                  (                                                                              H                            i                                                    ⁢                                                      W                                                          TX                              ,                              i                                                                                                      )                                            H                                                                      +                                                      σ                    i                    2                                    ⁢                  I                                            )                                      -              1                                                          (        1        )            
Equation (1) can be utilized in a case in which information on all downlink channels that may cause large interference can be estimated. In Equation (1), suffix i in each parameter denotes the number of the mobile communication terminal. In Equation (1), Ps is a scalar indicative of a transmission power per symbol from the desired base station for a mobile communication terminal #i. Hi is a channel matrix (channel impulse matrix) of the mobile communication terminal #i. In this channel matrix, the number of rows is the number of receiving antennas of the mobile communication terminal #i, whereas the number of columns is the number of transmitting antennas of the desired base station for the mobile communication terminal #i. In summary, this channel matrix is a channel matrix of downlink channels to the mobile communication terminal #i from the desired base station for the mobile communication terminal #i. WTX, i is a precoding matrix generated at the desired base station for the mobile communication terminal #i and used for downlink transmission from the desired base station to the mobile communication terminal #i. This precoding matrix has rows of which the number is the number of transmitting antennas of the desired base station for the mobile communication terminal #i, and columns of which the number is the number of the transmission layers, i.e., the number of the transmission streams transmitted from the desired base station for the mobile communication terminal #i. If the number of transmitting antennas of the base station is one, this precoding matrix is a scalar. (With this respect, Equation (1) can be also used in SIMO (single-input multiple-output).)
σi2 indicates a noise power at the mobile communication terminal #i, and σi is a standard deviation of the noise power. I is an identity matrix.
NUE is the sum of the total number of mobile communication terminals that receive downlink channels that may significantly interfere with the downlink channel received by the mobile communication terminal for which the receiving weights are to be calculated, and 1 (the number of mobile communication terminal for which the receiving weights are to be calculated). “Downlink channels that may significantly interfere with the downlink channel received by the mobile communication terminal” mean downlink channels that use the same frequency as that for the desired downlink channel.
Superscript H on the right side in the Equation (1) denotes complex conjugate transpose.
According to Equation (1), each mobile communication terminal can calculate receiving weights WMMSE, i not only on the basis of the channel matrix of the downlink channel from the desired base station for the mobile communication terminal to the mobile communication terminal, and the precoding matrix generated at the desired base station for the mobile communication terminal, but also on the basis of channel matrices of downlink channels of signals coming into the mobile communication terminal transmitted from desired base stations for other mobile communication terminals in order to send the downlink signals to other mobile communication terminals, and precoding matrices generated at other base stations for other mobile communication terminals for downlink transmission to other mobile communication terminals. In SU-MIMO, desired base stations for other mobile terminals are different from the desired base station for the mobile communication terminal for which receiving weights are to be calculated.
The mobile communication terminal can estimate the product of the channel matrix of the downlink channel from the desired base station to mobile communication terminal and the precoding matrix thereof, on the basis of a reference signal, which will be described later. In order to use Equation (1), the mobile communication terminal should know or estimate channel matrices and precoding matrices with respect to signals transmitted to other mobile communication terminals, i.e., interfering signals.
In a case in which information on all downlink channels that may cause large interference cannot be estimated, it is possible to calculate reception weights WMMSE, i for a mobile communication terminal that receives signals by using Equation (2) below, as an alternative IRC reception technique.WMMSE,i=PS(HiWTX,i)H(Ryy,i−1)T  (2)
In Equation (2), superscript T indicates transposition. Ryy,i is a covariance matrix of the received signal vector for the mobile communication terminal #i, and is calculated from Equation (3).
                              R                      yy            ,            i                          =                              1            M                    ⁢                                    ∑                              m                =                1                            M                        ⁢                                                            y                  i                                ⁡                                  (                  m                  )                                            *                                                y                  i                                ⁡                                  (                                      m                    T                                    )                                                                                        (        3        )            
In Equation (3), yi(m) is a vector of a signal received at the mobile communication terminal #i, in which m is the sample number (resource element number) of the received signal. Mobile communication terminal #i calculates the received signal vector yi(m) for each of receiving antennas of the mobile communication terminal #i. The sample number m is a combination of the number of the received subcarrier and the symbol number of the OFDM (Orthogonal Frequency Division Multiplexing) symbol. In Equation (3), the asterisk denotes conjugate, whereas T denotes transpose. M is the total number of samples used for averaging, and is freely determined. Thus, each mobile terminal processes vectors of samples of signals at respective receiving antennas of the mobile communication terminal, and averages the matrices obtained by the process, thereby obtaining the covariance matrix Ryy,i.
According to Equation (2), each mobile communication terminal can calculate the receiving weights WMMSE, i from the channel matrix of downlink channels from the desired base station for the mobile communication terminal to the mobile communication terminal, the precoding matrix for the mobile communication terminal generated at the desired base station, and received signal vectors. It is possible to estimate the product of the channel matrix of downlink channels from the desired base station to the mobile communication terminal and the precoding matrix thereof on the basis of the reference signal, which will be described later. Therefore, if Equation (2) is used, it is unnecessary to estimate channel matrices of interfering waves coming from interfering base stations for downlink signal transmission to other mobile communication terminals. However, for enhancing the ability to suppress interference by beams from other base stations, it is necessary to prepare many samples m (the number of resource elements) used for averaging in Equation (3).
In radio communication systems complying with LTE Release 10, different cell IDs are allocated to individual cells, i.e., individual base stations. Also, in Release 11, different cell IDs are allocated to individual base stations, except for remote radio heads (RRHs). For example, in the structure of FIG. 2, cell IDs 1, 2, and 3 are allocated to base stations 1, 2, and 3 (thus, cells 1a, 2a, and 3a), respectively. In FIG. 2, the base station 1 is the desired base station for the mobile communication terminal 10, whereas the base stations 2 and 3 are interfering base stations.
The mobile communication terminal 10 shown in FIG. 2 is informed of the cell ID of the desired base station 1, the number of transmitting antennas of the desired base station 1, the number of transmission layers transmitted from the desired base station 1 to the mobile communication terminal 10, and other information by control signals from the base station 1. However, in the radio system shown in FIG. 2 (radio system in which different cell IDs are allocated to all base stations), the mobile communication terminal 10 is not informed of cell IDs of interfering base stations. Therefore, it is difficult to conduct the IRC reception method in which information on the downlink channels from interfering base stations is estimated and used (i.e., the method for calculating receiving weights with the use of Equation (1)). This is because the mobile communication terminal 10 is not aware of cell IDs of interfering base stations, and therefore the terminal 10 cannot estimate directly information on the channels from interfering base stations. In an alternation, without knowing cell IDs of interfering base stations, it is possible to estimate blind information on the channels from interfering base stations, but in this case, accuracy of the calculated receiving weights are low.
Accordingly, in the following, another IRC reception method for calculating receiving weights with the use of Equation (2) will be discussed. As described above, in order to calculate receiving weights by Equation (2), a vector yi(m) of each sample (resource element) of the signal from each receiving antenna of the mobile communication terminal is processed, and the matrices obtained by this process are averaged in accordance with Equation (3), so that the covariance matrix Ryy,i is obtained.
The samples of the received signal can be selected in one of two manners described below. The selection manners will be described with reference to FIGS. 3 and 4. FIGS. 3 and 4 show mapping of signals on a resource block in OFDMA downlink transmission. Reference symbol RB designates a single resource block, and each square designates a resource element that is a minimum resource unit defined by a single subcarrier and a single OFDM symbol. The ordinate denotes frequency (subcarrier), whereas the abscissa denotes time (OFDM symbol).
The sample number (resource element number) m is specified by k and l in which k is the received subcarrier number (i.e., subcarrier index), and l is the OFDM symbol number (i.e., OFDM symbol index). The received signal vectors yi(m) of each resource element can be expressed by yi(k, l).
As shown in FIG. 3, in a manner for selecting received signal samples, for each of multiple subcarriers allocated to downlink transmission from the desired base station to the mobile communication terminal, multiple resource elements including resource elements for the data signal (resource elements of OFDM symbol numbers 3 through 13) are selected. That is to say, resource elements of OFDM symbol numbers 0 through 2 that never include the data signal are excluded. Additionally, the reference signal arranged in OFDM symbol numbers 4, 7, 8, 9, 10, and 11 shown in FIG. 3 may be excluded because the reference signal is transmitted without multiplication by precoding. Then, received signal vectors yi(k, l) of these selected resource elements are obtained, and the matrices yi(k,l)*yi(k,l)T is calculated. Furthermore, the matrices are averaged for the respective subcarriers, so that the covariance matrix Ryy,i is obtained for the respective subcarriers. Next, in accordance with Equation (2), for the respective subcarriers, receiving weights WMMSE, i is calculated. Thus, for the respective subcarriers, receiving weights are obtained.
As shown in FIG. 4, in another manner for selecting received signal samples, for a resource block, multiple resource elements including resource elements for the data signal (resource elements of OFDM symbol numbers 3 through 13) are selected. That is to say, resource elements of OFDM symbol numbers 0 through 2 that never include the data signal are excluded. Additionally, the reference signal arranged in OFDM symbol numbers 4, 7, 8, 9, 10, and 11 shown in FIG. 4 may be excluded because the reference signal is transmitted without multiplication by precoding. Then, received signal vectors yi(k, l) of these selected resource elements are obtained, and the matrices yi(k,l)*yi(k,l)T is calculated. Furthermore, the matrices are averaged over the single resource block, so that the covariance matrix Ryy,i is obtained for this resource block. Next, in accordance with Equation (2), for this resource block, receiving weights WMMSE, i is calculated. Thus, for the single resource block, receiving weights are obtained.